Hepatitis C virus (HCV) is the etiologic agent in the majority of cases of transfusion associated hepatitis (TAH) in the United States. HCV disease can also be contracted by intravenous or percutaneous blood exposure such as intravenous drug abuse, and occupational exposure to blood borne pathogens. Initiative to reduce the incidence of the disease include donor blood testing, and research on vaccines. The former has already resulted in a reduction of the risk of transmission of HCV. The development of an effective vaccine is hampered by the genetic diversity of the virus. As evidence for this diversity accumulates, it appears that strains of the virus ( at least 4 of which have been described in the U.S., Japan and Eurasia), while the second is the evolutionary change which occur in a strain over time. The later is the subject of this study. The investigators propose to obtain stored sera of 30 to 50 persons who have been referred for testing for HCV on more than one occasion. Positive screening tests are often repeated within six months. A comparative analysis of preselected structural regions in the genome for variation between the first and subsequent samples will be done on sample with positive confirmatory tests. It is hypothesized that the observed changes will not be random.